1. Field of the Invention
The present invention relates to a signal processing device, a signal processing method, particularly a signal processing device for processing a 1-bit digital signal obtained by a ΔΣ modulation.
The present invention also relates to a signal processing method to be applied to the above signal processing device.
The present invention further relates to a signal-level display device for displaying a signal level processed by the signal processing device.
2. Description of the Related Art
Super Audio Compact Disc (SACD) (trade name) has been known in the art. For example, in contrast to Compact Disc (CD) that store one sample as a multi-bit audio signal (16 bits) per each channel by the PCM system with a sampling frequency (fs) of 44.1 KHz.
In recent years, a method and device for recording/reproducing 1-bit audio signals into/from recordable/rewritable DVDs and semiconductor memories have been also proposed.
In many cases, such an audio device is provided with a level meter for confirming the volume of recording/reproduction sound.
In the PCM system, each sample represents a level (amplitude information) itself. Thus, it is easy to display the level of audio data on a level meter.
However, in the case of a 1-bit audio signal recorded on the above SACD, it realizes a high S/N ratio by a noise-shaping technique. Thus, the 1-bit audio signal has an additional frequency component out of a human audible frequency range (approximately 20 kHz). The component out of the audible range is almost removed by an analog LPF, so that there is a problem that a signal level in the audible range is not found even only in a 1-bit audio signal.
Therefore, an operation has been performed such that a signal component in the audible range is taken out by carrying out a low-pass filtering process on a 1-bit audio signal and the signal component is then converted into a multi-bit audio signal to take out a signal level.
Specifically, as shown in FIG. 1, the level generation of the related-art recording/reproducing apparatus is carried out as follows: At the time of both the recording and reproducing operations, a level generator 101 receives 1-bit audio signals D one by one and then subjects the input signal to a filtering arithmetic operation in a filter FLT 103 with a tap length of N to generate a level signal L, followed by supplying the level signal L to a level display unit 102.
As an example of the above level generation, FIG. 2A is a block diagram that represents a filter for carrying out a low-pass filtering process with a tap length of n. Input data D[i], which are sequentially entered and stored in memory FFs 2001 to 200n, are multiplied by low-pass filter factors C2011 to 201n and then added to an ADD unit 202 as much as the number of the taps N, thereby obtaining a low-pass output LIpf[i]. Subsequently, the low-pass output LIpf[i] is displayed on a level display unit. To perform the process, a 1-bit audio signal D does not use logical values (0/1) but actual values (−1/+1).
In addition, a moving-average filtering operation may be used as a method of realizing a low-pass filter in the absence of filter factors C. FIG. 2B is a block diagram illustrating a moving-average filter for performing a moving-average filtering operation. Input data D[i] sequentially inputted and stored in memory FFs 300l to 300n are added by an adder 301. The result of the addition is then divided by the tap length n (1/n). As a result, a moving average output Lma[i] is obtained and displayed on a level display unit. Since the low-pass characteristic of the moving-average filtering operation is uniquely defined, the flexibility of the moving-average filter is lower than that of a low-pass filter using arbitrary filter factors C. However, it may be sufficient for level generation. To perform the process, a 1-bit audio signal D does not use logical values (0/1) but actual values (−1/+1).
Furthermore, there is proposed a method of generating a level by carrying out a process equal to a moving-average filtering operation without performing a filtering operation to count the number of logical values 0 and 1 in a predetermined number N of 1-bit audio signals (see, for example, Japanese Published Patent Application No. 2003-16767).